1. Field of the Invention
The present invention relates generally to a backlight such as is used in backlighting a flat panel liquid crystal display (LCD), and more particularly to a backlight having an optical input arranged to provide a uniform light distribution to the LCD.
2. Description of the Related Art
Flat panel displays, such as liquid crystal displays or LCD's used in laptop computers, generally incorporate a backlighting system to illuminate a liquid crystal based display panel. Important requirements of the backlighting system are to provide a substantially uniform light distribution and to provide a sufficiently intense light distribution over the entire plane of the display panel. To accomplish these requirements, the backlighting system typically incorporates a light pipe to couple light energy from a light source to the LCD panel. In scattering backlighting systems, an array of diffusing elements are disposed along one surface of the light pipe to scatter light rays incident thereto toward an output plane. The output plane is coupled to the LCD panel, coupling the light rays into and through the LCD panel. While a scattering backlighting system offers the ability, by controlling the distribution of the scattering media on the scattering surface, to control the light distribution, it does not offer an ability to control the angle of light distribution. Much of the light energy produced by the backlighting system is wasted because it is scattered in directions that are not useful to a viewer or user of the LCD display. Because much of the light energy is not directed to the user and thus wasted, scattering backlighting systems lack the desired light energy intensity or brightness.
Non-scattering backlighting systems offer the advantage that both the light distribution and the angle of distribution may be controlled. Thus, the light energy may be directed in a way to make more efficient use of the available light energy. For example, the light energy may be directed so that substantially all of the light energy is emitted toward the user. A term often used to describe non-scattering backlighting systems is "deterministic" because the output point of a light ray is known based upon its input position. Thus, it may be said that a non-scattering backlighting system correlates the rays of input light energy and the rays of output light energy.
This correlation is advantageously used in the design of a backlighting system to ensure that a majority of the light energy is directed to the user. The correlation of input light rays to output light rays in a non-scattering backlighting system may also lead to a potential disadvantage arising from imaging at the light input appearing at the light output. If there is any distortion of the light energy at the input, this distortion will also appear at the output. The distortion may result from roughness or discontinuity in the light source or the input optics. Generally, such distortions will result in an area of non-uniform light intensity or a shadow in the output. Another source of distortion is the construction of the walls of the light pipe perpendicular to the light source. These walls must be made extremely smooth and flat or else they result in a distortion or shadow in the output.
A particular distortion that is observed in non-scattering backlighting systems is the formation of a diagonal line across the output plane of the backlight. It has been observed that distortions at the corner or interface between each side wall of a light pipe and the input surface of the light pipe are imaged into the output as a dark, diagonal line. The distortions in each corner are due to manufacturing limitations in the construction of the light pipe. While it is possible to polish and smooth the surfaces to reduce the appearance of such a distortion, these processes are time consuming and labor intensive and, therefore, are impractical in mass production of light pipes.
Additionally, it has been observed that non-uniformity in the direct output of the light source may cause distortions and shadows in the output. More particularly, a cold cathode fluorescent light (CCFL), for example, has inherent dim regions adjacent the electrodes at each end of the CCFL tube. These dim regions are areas where the light output from the CCFL is not uniform, and the light output is substantially diminished as compared to a center portion of the tube. Not only do these dim areas image into the output plane, but they also contribute to and exacerbate the appearance of the diagonal line.
Other light sources may be utilized in an LCD light pipe such as light emitting diodes (LED's), incandescent bulbs, laser diodes, and virtually any other point light source. These light sources each typically exhibit some non-uniformity in the light output energy as well, creating a distortion problem in the LCD output.
Co-pending and commonly assigned U.S. patent application Ser. No. 09/137,549, pending, entitled "Light Pipe for a Backlighting System" discloses a light pipe construction which addresses the diagonal line problem. The disclosed light pipe includes extensions beyond the perimeter of the output surface dimensions for use with an oversized light source. The extensions and elongate bulb correct the diagonal line problem but disadvantageously increase the size of the light pipe.